Method of on-line coating of a film on the inner walls of the reaction tubes in a hydrocarbon pyrolysis reactor

ABSTRACT

A method of on-line coating a coat film on the inner wall of a reaction tube in a hydrocarbon pyrolysis reactor for preventing the formation and the deposit of coke on the inner walls. This method comprises the steps of vapor depositing a mixed solution of a metal alkoxide and a chromic compound on the inner walls concurrently with introducing a carrier at a flow rate of 1-5000 kg/hr/coil at a temperature of 600-900° C. under a pressure of 0-3 kg/cm 2  to form a buffer layer on the inner walls; and vapor depositing a metal alkoxide as a barrier on the buffer layer; and vapor depositing an alkali metal/alkaline earth metal compound alone or mixed with metal alkoxide as a decoking layer on the barrier. A decoking layer may further be provided on the diffusion barrier.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method of on-line coating acoat film on the inner walls of the reaction tubes in a hydrocarbonpyrolysis reactor so that the formation and the deposit of coke on theinner walls can be prevented and inhibited. More particularly, theinvention relates to a method of on-line coating an inorganic coat filmon the inner walls of the reaction tubes to prevent the formation andthe deposit of coke on the inner walls, to remove residual cokecontinuously, and to inhibit the carburization and deterioration ofmetals on the inner walls so that the operational duration of thereactor can be extended.

[0003] Reactors for use in the pyrolysis of hydrocarbons are commonlycomposed of a heating furnace and a series of tubular reactors, and areused in producing olefins such as ethylene, propylene and the like, bysupplying steam and a hydrocarbon feedstock into the tubular reactorsconcurrently at an elevated temperature of above 800° C. in gaseousphase to induce the hydrocarbon feedstock pyrolysed. During thepyrolysis reaction, coke is formed as a by-product from thedehydrogenation of hydrocarbon by way of the catalytic and/or pyrolyticreaction. Catalytic coke is formed from catalytic reaction between ahydrocarbon and metals such as nickel and iron, which are present on thesurface of the tubular reactor. Dehydrogenation of light olefins such asacetylene produces gaseous cokes, and dehydrogenation of heavy aromaticmaterials yield condensed coke.

[0004] These gaseous and condensed cokes are collectively referred to asa pyrolytic coke. As the pyrolysis reaction runs, the coke aggregatesand accumulates on the inner walls of the reactor, alone or through acooperative trap action.

[0005] Such aggregation or accumulation of the coke on the inner wall ofthe reactor tubes interferes with the flow of fluids in the pyrolysisreactor, causing an increase in the pressure drop between the frontaland the postal zones of the reactor, and deteriorates the efficiency ofthe heat transfer through the inner walls. This results in the reductionof the yields of the main product and the increase of the energyconsumption. Carburization may also occur into the metals of which thereaction tubes are fabricated, reducing the durability of the reactortubes.

[0006] 2. Disclosure of the Prior Art

[0007] Therefore, when the coke is accumulated on the inner walls of thereaction tubes to a certain level, the operation of the reactor must beshut down to eliminate the coke accumulated. The amount of theproduction loss and the energy consumed to eliminate the accumulatedcoke are considerably high. Thus, many approaches have been proposed toprevent and inhibit the formation and deposit of the catalytic coke onthe inner walls of a hydrocarbon pyrolysis reactor and to extend thecycle of operating the reactor.

[0008] Such approaches include: a method of using a specific alloy as areactor tube material; a method of continuously injecting a certainchemical such as sulfur, an alkaline metal salt, an alkaline earth metalsalt, phosphor, boron, cerium, lanthanum, molybdenum, or the like into ahydrocarbon feedstock; a method of pre-treating the inner surfaces ofthe reaction tubes with tin and silicone, aluminum, and phosphor; amethod of allying a ceramic film onto the inner walls of a tubularreactor by molten-coating the inner walls with an alkaline earth metalcompound; a method of physical vapor deposition of a mixture of a metaland ceramics on the inner walls of the reaction tubes; a method ofchemical vapor deposition of silicone ceramics on the inner walls of thereaction tubes; and so forth.

[0009] U.S. Pat. Nos. 4,889,614 and 5,358,626 disclose a process for thegasification of coke into carbon monoxide or carbon dioxide bycontinuously injecting an alkaline metal or alkaline earth metal salt asa catalyst into a pyrolysis reactor during the pyrolysis of thehydrocarbon. However, this process has a drawback in that a considerableamount of the catalyst may be entrained into and accumulated on therecovery section.

[0010] WO 97/41275 discloses a method of forming a protective oxide filmby coating a mixture of chromium, aluminum and silicon on fresh reactiontubes in a thickness of about 300 μm by way of a physical vapordeposition and then oxidizing the resultant coat film. This method,however, requires a separate step of coating a mixture of metals on thefresh tubes by an off-line method. Once the film has been worn down, itis impossible to re-coat the tubes without replacing the used tubes withnew ones.

[0011] U.S. Pat. No. 4,099,990 discloses a method of vapor depositing asilica film having a thickness of about 2 μm onto a metallic reactiontube using tetraethoxysilane as a vapor deposition material and steam orcarbon dioxide as a carrier. A test for the pyrolysis of ethane in theresultant tubes demonstrated that the amount of the coke accumulatedcould be reduced by about 80% at a temperature of 850° C. or less ascompared with the uncoated tubes, while at a temperature of above 850°C., no improvement was obtained. In connection with this method,however, it is necessary to consider the appropriate coating thicknessand the vapor deposition conditions for performing the function of thesilica film as a barrier and to provide a measure by which themechanical/thermal strength of the film can be secured. If the silicafilm is too thin, it does not perform its function as a barriercompletely and may be easily worn out by carburization or oxidationduring decoking. On the other hand, if the film is too thick, it is aptto peel off or rupture due to the difference in thermal expansionbetween the matrix metal and the silica film. Further, this patentneither suggests nor teaches clearly how to remove the accumulated coke,even though it is possible to reduce the accumulation of the coke tosome extent by covering the tube surfaces with an inactive inorganicoxide film to inhibit the catalytic reaction of the metallic componentsby which the formation of coke is accelerated.

DETAILED DESCRIPTION OF THE INVENTION

[0012] According to the present invention, it has been discovered thatby coating the inner walls of the reaction tubes in a hydrocarbonpyrolysis with an inorganic film to prevent a catalytic reaction betweena hydrocarbon feedstock and a metal such as nickel and iron, it ispossible to lower the formation and the deposit of coke on the innerwall and inhibit the carburization and deterioration phenomena of themetals due to the coke.

[0013] Accordingly, the object of the invention is to provide a methodof on-line coating the inner walls of the reaction tubes in ahydrocarbon pyrolysis reactor to prevent the formation and the depositof coke on the inner walls.

[0014] The above object of the invention can be achieved by an on-linemethod of providing a coat film on the inner wall of a reaction tube ina hydrocarbon pyrolysis reactor, which comprises the steps of vapordepositing a mixed solution of a metal alkoxide and a chromic compoundon the inner wall concurrently with introducing a carrier at a flow rateof 1-5000 kg/hr/coil at a temperature of 600-900° C. under a pressure of0-3 kg/cm² to form a buffer layer on the inner walls; and vapordepositing a metal alkoxide as a diffusion barrier on the buffer layer;and then vapor depositing an alkali metal/alkaline earth metal compoundalone or mixed with metal alkoxide as a decoking layer to obtain acontinuously formed coat film.

[0015] In this connection, it is noteworthy to understand that thediffusion barrier can be vapor deposited directly on the inner wallsunder the same conditions as mentioned above, without providing thebuffer layer, and that a decoking layer can then be vapor deposited onthe diffusion barrier and/or the buffer layer.

[0016] The above and other objects as well as advantages of theinvention will be more apparently recognized with reference to thedescription hereinafter and the accompanying drawings wherein:

[0017]FIG. 1 illustrates a schematic diagram of an apparatus for coatinga buffer layer, and/or a diffusion barrier, and/or a decoking layer onthe inner walls of a reaction tube, and performing a test for thepyrolysis reaction of a hydrocarbon using the tube so coated, accordingto the invention; and

[0018]FIG. 2 is graph showing a change in the pressure drop at the inletof the reactor used in Example 4 of the invention, compared with thepressure drop at the inlet of a conventional hydrocarbon pyrolysisreactor.

[0019] The present invention relates to a method of on-line coating acoat film for preventing the formation and the deposit of coke on theinner walls of the reaction tubes in a hydrocarbon pyrolysis reactor andpreventing the carburization or deterioration of metals on the innerwalls. According to the invention, it is essential to provide, in abatch process, an appropriate layer that can serve as a diffusionbarrier on the inner walls of the reaction tubes in a hydrocarbonpyrolysis reactor; and a buffer layer which can obtain a solid coat filmhaving good chemical/thermal strength so that better durability can beachieved; and a decoking layer which can minimize the amount of thegaseous coke deposited and condensed on the inner walls, which are notsuppressed easily by the barrier. In order to make the coat film formedaccording to the invention commercially useful, it is important that thefilm should have sufficient mechanical and thermal strength to resistagainst the operating conditions. It is also important that it is ableto eliminate any adverse effects which may occur in the postal steps ofthe reaction process, and that it can be re-coated on the inner wallswhen necessary.

[0020] As well known in the art, the term “on-line coating” used hereinmeans that the method of the invention can be performed during theoperation of the hydrocarbon-pyrolysis process without cooling down theprocess and removing the reaction tubes.

[0021] The inventors have discovered a thickness of the barrier suitablefor performing the invention by subjecting it to a pyrolysis test. Theproblems associated with the peeling off or rupture of the diffusionbarrier (inorganic coat film) due to the difference in the coefficientof thermal expansion between the metal and the barrier have also beeneliminated by forming a buffer layer using a mixture of the buffercomponents.

[0022] The residual cokes not suppressed by the barrier have also beenremoved by forming a decoking layer. Also, the problems of adverseeffects on the recovery section have been addressed by sending thefirnace effluents to a decoking drum.

[0023] Further, according to the invention, the reaction tubes candirectly be coated while being hot and, therefore, it is possible tore-coat the tubes in a hot state during the operation of the reactor.

[0024] Particularly, in performing the invention, after the coke iscombusted in a hydrocarbon pyrolysis reactor equipped with a series oftubular reaction tubes, by way of a carrier of 1-5000 kg/hr/coil at600-900° C. under 0-3 kg/cm², a metal alkoxide is carried into the tubesin an amount of 0.001-10%, preferably 0.05-1.0% based on the weight ofthe carrier introduced. Then, the alkoxide absorbs the heat for aresidence time of 0.5-2 seconds, and is then decomposed and deposited onthe inner walls of the tubes to form a diffusion barrier. Where theamount of the alkoxide introduced is below 0.001% by weight, it requiresa long period of time to be coated to a desired thickness. Where theamount exceeds 10% by weight, the resultant particles may deterioratethe physical properties of the coat film so formed. The thickness of thecoat film is preferably in the range of 4-12 μm. If the thickness isbelow 4 μm, the film does not serve as a barrier. If the thicknessexceeds 12 μm, the film tends to peel off or rupture as the temperaturevaries.

[0025] Upon completion of the deposition of the coat film, thetemperature is gradually elevated until it reaches the temperature ofhydrocarbon pyrolysis by 200° C. or less per hour. In this process, theorganic groups remaining in the coat film are removed to form a solidinorganic coat film. Non-limiting metal alkoxides suitable for adeposition material include volatile silicone alkoxides such astetraethoxysilane and tetramethoxysilane; titanium alkoxides such astitanium tert-butoxide, titanium ethoxide, and titanium isopropoxide;and aluminum alkoxides such as aluminum acetylacetonate and aluminumisopropoxide. These are used alone or in combinations thereof.Non-limiting carriers include nitrogen, helium, argon, carbon dioxide,air, oxygen, and steam. Among them, steam is more preferred in thepyrolysis in view of saving production costs.

[0026] The alkoxide may be introduced into the reactor tube, forexample, by diluting it in the carrier. The dilution is carried out bybubbling the carrier through the alkoxide or by pumping the alkoxidedirectly into the carrier. Practically, the latter method is morepreferred.

[0027] However, the inorganic coat film formed of such a metal alkoxideis apt to easily peel off or rupture from thermal shock because itsthermal expansion coefficient is just about 1/20 as compared with thatof the metallic tubular reactor. Therefore, it is desirable to have amedium layer, namely, a metal oxide buffer layer sandwiched between theinner wall of the metallic reactor tube and the inorganic coat film toreinforce the mechanical/thermal strength of the entire coat film. Theoxide buffer layer can be provided by depositing a mixed solution of ametal alkoxide and a chromium compound on the inner walls of thereaction tubes. The chromium compounds include chromium acetylacetonate, chromium carbonyl, chromic (III) 2-ethylhexanoate, andchromic (III) hexafluoroacetyl acetonate. Preferably, chromium acetylacetonate [Cr(C₅H₇O₂)₃, FW 349.33] is used, which has a boiling point ofabout 340° C. and is sufficiently volatile.

[0028] The above chromium compounds are present in solid form at roomtemperature and thus are needed to dissolve them in an adequate organicsolvent such as tetrahydrofuran, toluene, and so forth. The toluene mayact as a precursor producing coke during pyrolysis at a hightemperature. Therefore, the use of toluene is avoided when possible andtetrahydrofuran is preferably used in the invention. Concurrently withintroducing the carrier into the reactor tubes, a metal alkoxide and achromium compound are injected in an amount of 0.001-10% by weight andin an amount 0.001-1.0% by weight, respectively, based on the totalweight of the carrier, resulting in a buffer layer having a 1-5 μmthickness directly on the inner walls of the reactor tubes. Preferably,the metal alkoxide and the chromium compound are introduced in a mixturein an amount of 0.005- 1.0% by weight based on the total weight of thecarrier. When the metal alkoxide is used in an amount of less than0.001% by weight, it is insufficiently transferred to the outlet of thereactor, or requires much time to obtain a suitable thickness of thebarrier film. When the alkoxide is used in an amount of above 10% byweight, it will be wasteful due to its low deposition yield and thephysical properties of the resultant coat film will be degenerated dueto the formation of particles. If the amount of the chromium compoundused is less than 0.001% by weight, there will be little or no effect.If it is used in an amount of above 1.0% by weight, it will also bewasteful. Further, if the thickness of the buffer layer is less than 1μm, satisfactory buffering effects cannot be obtained, and if above 5μm, further improvement cannot be expected.

[0029] Meanwhile, the provision of a diffusion barrier on the surfacesof the reactor tubes can inhibit a catalytic reaction of the metalcomponents that may cause the coke formation. However, the gaseous orcondensed coke cannot be prevented from being deposited on the barrier.In addition to the diffusion barrier, a mixture of an alkoxide and analkaline metal or an alkaline earth metal oxide may be deposited on thebarrier to form a decoking layer thereon. This decoking layer can givecoke gasification ability to the barrier.

[0030] In providing an alkaline metal or alkaline earth metal oxide filmon the reactor tubes as a decoking layer in accordance with the on-linecoating method of the invention, it is important to select adequateprecursors depending upon the operating conditions, such as the vapordeposition temperature and the types of the carriers. Those precursorsinclude organic compounds such as alkoxides, beta diketonates,alkylates, carboxylates, and inorganic compounds such as nitrates andcarbonates. Examples of the alkaline metal/alkaline earth metalcompounds include potassium acetyl acetonate, potassium tetramethylheptanedionate, potassium acetate, calcium acetyl acetonate, calciumtetramethyl heptanedionate, calcium acetate, magnesium acetyl acetonate,magnesium tetramethyl heptanedionate, magnesium acetate, barium acetylacetonate, barium tetramethyl heptanedionate, barium acetate, lithiumacetyl acetonate, lithium tetramethyl heptanedionate, and lithiumacetates. These compounds may be used alone or in combinations thereof.

[0031] The amount of alkaline metal/alkaline earth metal compounds usedis 0.001-10% by weight, preferably 0.05-1.0% by weight with respect tothe total amount of the carrier introduced. Beyond this range, thedeposition time may be increased, resulting in lowering the depositionyields and degenerating the physical properties of the desired coatfilm. The thickness of the coat film deposited is preferably 0.1-2 μm.If the thickness is not within this range, a sufficient cokegasification effect cannot be obtained, or an undesirable coat film isapt to be formed.

[0032] As described above, the method according to the invention can beperformed at a high temperature, and thus a pure inorganic oxide filmcan be deposited on the inner walls of the reaction tubes by minimizingthermal shock. Further, in the invention, there are no limitations inthe surface conditions and the term of use of the reaction tubes to becoated. Therefore, whenever the coat film has been worn out, it ispossible to re-coat the film on the inner walls of the tubes during theoperation of the reactor without replacing the tubes with new ones.

[0033] In addition, sufficient mechanical/thermal strength can beobtained by introducing a buffer layer between the tube metal and thebarrier to provide a coat film having an appropriate thickness. Also,residual coke can be removed by impregnating a catalyst for gasifyingcoke on the diffusion barrier without adverse effects on the recoverysection.

[0034] According to the invention, it has been confirmed that in orderto assure the function of the coat film for inhibiting the carburizationof the metal components in the inner walls of the reaction tubes, thethickness of the coat film must be about 4 μm. In performing the methodof the invention, the increasing rate of the pressure at the inlet ofthe reactor is reduced to below ½; thus, the operating term of thereactor can be considerably extended.

[0035] The present invention will be described in greater detail bymeans of the following non-limiting examples.

EXAMPLE 1

[0036] In this example, an experimental test for applying a buffer layerbetween the inner walls of a metallic tube used in a hydrocarbonpyrolysis reactor and a diffusion barrier, namely a silica film coatformed thereon, to compensate for the thermal expansion difference andincrease the mechanical/thermal strength between the metallic tube andthe barrier was performed. A mixture of chromium oxide and silica wasused as a buffer layer. As material for the vapor depositions of thechromium oxide and the silica, chromium acetyl acetonate and tetraethoxysilane were each used. For the purpose, 1.2 g of chromium acetylacetonate was dissolved in 36 ml of tetrahydrofuran.

[0037] As shown in FIG. 1, a tubular tube (0.68 cm in inner diameter and69 cm in length) made of a high nickel alloy was installed within theheating furnace. The tube was adjusted at an inlet temperature of 620°C. and at an outlet temperature of 750° C. Then, steam preheated at 450°C. was passed through the tube at a flow rate of 100 g/h for 8 hours,together with 0.5% of tetraethoxysilane and 0.01% of chromium acetylacetonate, based on the total weight of the steam, to coat the innerwall of the tube. Observation under a scanning electron microscope ofthe resultant coat film showed that a resultant buffer layer of a metaloxide having a thickness of about 3-4 μm was formed.

[0038] After the buffer layer was formed, tetraethoxysilane wascontinued to be injected into the tube in an amount of 0.6% by weightfor 10 hours, to evaluate the resultant coat film. After the coatingoperation was completed, the resultant coat film was examined under ascanning electron microscope with respect to its morphology andthickness. The diffusion layer was confirmed to be a solid film having athickness of 7-11 μm.

[0039] To test the thermal strength of the coat film, a test piece wasmaintained for 60 hours at 1000° C. and then slowly cooled. Observationunder a scanning electron microscope confirmed no loss occurred in thefilm.

EXAMPLE 2

[0040] In this example, another test was carried out to evaluate theability of preventing and reducing the formation and deposit of coke onthe inner walls of a reaction tube in a hydrocarbon pyrolysis reactor. Asample reaction tube which is made of HK4M (Cr: 25% and Ni: 25%) wasinstalled in the reactor as shown in FIG. 1, and the inner wall of thetube was then coated according to the same procedures described as inExample 1. Ethane was subject to pyrolysis in the reactor under thereaction conditions given in a table below. After the pyrolysisreaction, a scanning electron microscope examination showed that theformation of coke on the inner walls was dramatically reduced ascompared with the uncoated tube. TABLE Feedstock Ethane InletTemperature (° C.) 600 Outlet Temperature (° C.) 845 Reactor PressureAtmosphere Weight Ratio of Steam/Ethane 0.3 Conversion of Ethane (%) 80Operation Period (hours) 2

EXAMPLE 3

[0041] In this example, a solution of an inorganic alkaline metalcompound, calcium acetyl acetonate was further injected onto the silicadiffusion barrier for 7 hours in an amount of 0.067% by weight withrespect to the amount of the steam concurrently injected, to form adecoking layer on the diffusion barrier. The formation of coke wascontinuously eliminated during operation. The solution was prepared bydissolving 1 g of calcium acetyl acetonate in a mixture of 50 g ofmethanol and 0.5 g of 60% nitric acid. An X-ray analysis of energyscattering type detected 9.0% by weight of calcium. To compare theefficiency of removing the coke by the decoking formed on the barrierwith that by a combination of the buffer layer and the diffusionbarrier, the same test as described in Example 2 was performed, and itwas then observed under an scanning electron microscope. As a result, nocoke formation was observed.

EXAMPLE 4

[0042] The results obtained from the tests done above were applied to acommercial pyrolysis reactor (KBR Millisecond pyrolysis reactor).Immediately after the combustion process of coke was completed, theinlet and the outlet of the reactor were adjusted at 620° C. and 730°C., respectively. Then, 74 kg/h per tube of the steam preheated to 620°C. was injected into the reaction tubes in the reactor, together with0.002% by weight of chromium acetyl acetonate and 0.2% by weight oftetraethoxysilane, for 8 hours, to have the inner walls of the reactiontubes coated. Upon completion of the coating operation, a feedstock,naphtha was charged into the reactor after the temperature was elevatedto a temperature for carrying out the pyrolysis of the naphtha by 60° C.per hour. The level of the coke formed was observed by monitoring thepressure change at the inlet of the reactor. The pressure changes beforeand after coating the inner walls of the reactor tubes are plotted inFIG. 2. As can be seen from FIG. 2, the increasing rate of the pressureafter coating is about half of that before the coating.

[0043] As described above, according to the invention, it is possible toprevent the production of coke during the pyrolysis reaction ofhydrocarbons in a pyrolysis reactor and remove the coke through thegasification thereof. This allows the cycle run length of the reactor tobe extended. Accordingly, it is also possible to save the costs ofcombustion of the coke and to increase the production capacity of thetarget products. Further, according to the invention, carburization anddeterioration of metals on the reactor tubes by the barrier function ofthe diffusion barrier can also be inhibited. This extends the life ofthe reactor tubes. The invention has another advantage in that a coatfilm having an improved mechanical/thermal strength can be obtainedwithout causing any adverse effects during the postal step of theprocess involved, and that the coat film can be intermittently coated onthe inner wall of a reaction tube in an on-line method whenever desiredduring the process, even if the initial coat film was not permanentlymade.

[0044] It will be apparently understood, of course, that various changesand modifications can be made in the embodiments of the inventionillustrated and described herein without departing from the spirit andthe scope of the invention as defined by the appended claims.

What is claimed is:
 1. A method of on-line coating a continuousinorganic coat film on an inner wall of a reaction tube in a hydrocarbonpyrolysis reactor for preventing the formation and deposit of cokes onthe inner wall, comprising: vapor depositing a metal alkoxide as adiffusion barrier on the inner wall, concurrently with introducing acarrier into the reaction tube at a flow rate of 1- 5000 kg/hr/coil at atemperature of 600-900° C. under a pressure of 0-3 kg/cm².
 2. The methodof claim 1, wherein a buffer layer is provided directly on the innerwall of the reaction tube, before forming the diffusion barrier, thebuffer layer being formed by vapor depositing a mixed solution of ametal alkoxide and a chromium compound on the inner walls, concurrentlywith introducing a carrier into the reaction tube at a flow rate of1-5000 kg/hr/coil at a temperature of 600-900° C. under a pressure of0-3 kg/cm².
 3. The method of claim 1 further comprising forming adecoking layer onto the inorganic coat film by vapor depositing amixture of an alkoxide and an alkaline metal or alkaline earth metaloxide, concurrently with introducing a carrier into the reaction tube ata flow rate of 1-5000 kg/hr/coil at a temperature of 600-900° C. under apressure of 0-3 kg/cm².
 4. The method of claim 1, wherein the carrier isone selected from the group consisting of nitrogen, helium, argon,carbon dioxide, air, oxygen, and steam.
 5. The method of claim 1,wherein the inorganic coat film is deposited by introducing the metalalkoxide in an amount of 0.001-10% by weight based on the total amountof the carrier into the reactor.
 6. The method of claim 1, wherein theinorganic coat film is 4-12 μm in thickness.
 7. The method of claim 2,wherein the metal alkoxide and the chromium compound are each introducedinto the reaction tube in an amount of 0.001-10% by weight, based on thetotal weight of the carrier.
 8. The method of claim 7, wherein theamount is 0.05-1.0% by weight.
 9. The method of claim 2, wherein thebuffer layer is 1-5 μm in thickness.
 10. The method of claim 1, whereinthe metal alkoxide is one selected from the group consisting oftetraethoxysilane, tetramethoxysilane, titanium tert-butoxide, titaniumisopropoxide, aluminum acetyl acetonate, and aluminum isopropoxide, anda combination thereof.
 11. The method of claim 2, wherein the chromiumcompound is one selected from the group consisting of chromium acetylacetonate, chromium carbonyl, chromic (III) 2-ethylhexanonate, andchromic (III) hexafluoroacetyl acetonate, and a combination thereof. 12.The method of claim 3, wherein the alkaline metal or alkaline earthmetal is one selected from the group consisting of potassium acetylacetonate, potassium tetramethyl heptanedionate, potassium acetate,calcium acetyl acetonate, calcium tetramethyl heptanedionate, calciumacetate, magnesium acetonate, magnesium tetramethyl heptanedionate,magnesium acetate, barium acetyl acetonate, barium tetramethylheptanedionate, barium acetate, lithium acetyl acetonate, lithiumtetramethyl heptanedionate, and lithium acetate, and a combinationthereof.
 13. The method of claim 3, wherein the alkaline metal oralkaline earth metal compound is introduced into the reactor in anamount of 0.001-10% by weight based on the total amount of the carrier.14. The method of claim 3, wherein the barrier is 0.1-2 μm in thickness.